Process for utilizing vertical autoclave in polymerization of vinyl chloride

ABSTRACT

A vertical autoclave for carrying out operations involving a medium having a liquid phase and/or a solid phase, comprising: 
     (a) a concave autoclave bottom; 
     (b) a shaft along a vertical axis of said autoclave, said shaft passing through said bottom of said autoclave; and 
     (c) stirrer means for driving said medium with a centripetal motion about said concave autoclave bottom.

This is a division of application Ser. No. 850,286, filed April 10,1986, now U.S. Pat. No. 4,729,878.

BACKGROUND OF THE INVENTION

The present invention relates to a vertical autoclave which can be usedto carry out any operations involving a medium comprising a liquid phaseand/or a solid phase and especially for the bulk manufacture of vinylchloride-based polymers and copolymers. It also relates to a process forthe bulk manufacture of vinyl chloride-based polymers and copolymers,implemented with the use of the said autoclave.

The bulk polymerization of a monomer composition based on vinyl chloridestarts with a liquid monomer composition. Because of the insolubility ofthe polymer or of the copolymer in the monomer composition, the reactionproduct separates out in a solid state during the polymerization, in theform of particles which are kept suspended in the liquid monomercomposition by the application of agitation; as by stirring, to thereaction medium. When the polymer or copolymer content in the reactionmedium reaches a value of the order of 15% by weight, its consistencybecomes similar to that of stirred curdled milk. This consistencyincreases until the monomer composition no longer forms a continuousliquid phase, and this occurs as soon as the polymer or copolymercontent in the reaction medium reaches a value of the order of 25% byweight, beyond which value the reaction medium is in a pulverulentstate. The reaction medium is maintained in this state by virtue of thecontinuation of the agitation, as by stirring, which prevents thereaction product from setting solid, until the polymer or the copolymeris obtained in a pulverulent state at the end of polymerization, afterthe unreacted monomer composition has been degassed.

In the present application, the expression "monomer composition based onvinyl chloride" or more briefly, "monomer composition" refers to vinylchloride by itself or mixed with at least one other monomer which can becopolymerized with vinyl chloride. The said monomer composition based onvinyl chloride contains at least 70% by weight of vinyl chloride. Asmonomers which can be copolymerized with vinyl chloride there may bementioned, especially, vinyl acetate and low molecular weight olefinssuch as, for example, ethylene and propylene.

Bulk polymerization of a monomer composition based on vinyl chloride isgenerally carried out in the presence of at least one polymerizationinitiator which generates free radicals such as, for example, an organicperoxide or an azo compound.

When the required degree of conversion of the monomer composition hasbeen obtained, the polymerizate obtained is subjected to a degassingtreatment, carried out with stirring, and intended to separate theunreacted monomer composition from the polymers or copolymers.

The pulverulent polymer or copolymer obtained after degassing theunreacted monomer composition is brought to atmospheric pressure bymeans of an inert gas, such as nitrogen, before its exposure to openair, which is generally followed by a screening operation.

The purpose of the screening operation, which is carried outcontinuously on an industrial scale, is to separate the correct product,defined as that which passes through a sieve with mesh openings of agiven size, chosen as a function of the particle size distribution ofthe polymer or of the copolymer and of the use for which it is intended,from the coarse product consisting substantially of agglomerates ofresin particles, of lower commercial value and defined as that which isretained on the said sieve.

It has already been proposed to carry out the manufacture of vinylchloride-based polymers and copolymers by bulk polymerization orcopolymerization in two stages, carried out in separate units by meansof a process consisting of implementing the polymerization orcopolymerization operations by ensuring that the rate of stirring of thereaction medium is as high as possible during a first stage, this beinguntil a degree of conversion of the monomer composition of the order of7% to 15% and preferably of the order of 8% to 12% is obtained, and thenreducing the rate of stirring during the second stage of the reaction toa value which is as low as possible, but which still remains sufficientto ensure good heat exchanges within the reaction medium, this beingdone until the end of the said reaction. According to these methods, thetwo stages, the first carried out with high-turbulence stirring, and thesecond at a slow rate of stirring, are implemented in differentautoclaves, the said stages, known as the prepolymerization stage in thecase of the first one and as the final polymerization in the case of thesecond one, taking place in suitable units known as prepolymerizers andpolymerizers. Also according to these methods, the second step iscarried out using a reaction medium consisting either only of themonomer/polymer composition produced in the first stage ofpolymerization, or of the said monomer/polymer composition and anadditional monomer composition based on vinyl chloride, identical ordifferent from that used during the first stage, and one or morepolymerization initiators.

This process and its alternative methods of implementation have beendescribed in detail in French Patents and Certificates of Addition Nos.1,382,072, 84,958, 84,965, 84,966, 85,672, 89,025 and Nos. 1,436,744,87,620, 87,623, 87,625 and 87,626.

According to a particular method of implementing the process, the finalpolymerization operation is carried out in a vertical autoclave,described in French Patent No. 73/05,537, published under No. 2,218,350,equipped with a stirring device comprising a stirrer consisting of atleast one arm matching the shape of the concave bottom of the autoclaveand connected to a shaft passing through the bottom of the autoclavealong its axis.

The definitions of values, concepts, and expressions used in the presentapplication are given below, to characterize, on the one hand, theautoclave and, on the other hand, any stirrer of the type consisting ofat least one arm matching the shape of the concave bottom of theautoclave and connected to a shaft passing through the bottom of theautoclave along its axis.

1. The "bottom part of the autoclave" means the volume swept by rotatingthe stirrer about the axis of the autoclave.

2. R denotes the radius, called "radius of the autoclave", of the crosssection of the internal surface of the autoclave in the planeperpendicular to its axis passing through the radius of the bottomflange of the autoclave.

3. r denotes the radium, called "radius of the shaft", of the crosssection, in a plane at the right angles to the axis of the autoclave, ofthe surface of the volume swept by rotating the shaft about the saidaxis.

4. Σ₁ denotes the cylindrical surface of revolution, having theautoclave axis as its axis, whose cross section in a plane (P) at rightangles to the axis of the autoclave at any point O, has a radius equalto 1.05 r.

5. Σ₂ denotes the cylindrical surface of revolution, having theautoclave axis as its axis, passing through the point or points of thesurface of the arm which are the furthest away from the said axis.

6. d denotes the radius of the cross section of the surface Σ₂ in theplane (P).

7. The "leading line" and "trailing line" of the arm mean the front partand the rear part, respectively, of the outline of the face of the arm,viewed from below, each part being bounded by its points of meeting withthe surfaces Σ₁ and Σ₂.

8. The "front" part and "rear" part mean the front part and the rearpart, respectively, in the considered direction of rotation of thestirrer.

9. (C) and (C') denotes the corresponding projections DE and D'E' of theleading line and of the trailing line of the arm in the plane (P).

10. Points D and D', known as the "central" points of the projections(C) and (C'), respectively, are situated at a distance from the point Owhich is equal to 1.05 r.

11. The points E and E', which are separate or which coincide, called"peripheral" points of the projections (C) and (C'), respectively, aresituated at a distance d from the point O.

12. e denotes the distance from the central point D of the projection(C) to the central point D' of the projection (C').

13. The "angle of curvature" at any point F of the projection (C) meansthe angle O X, F Y formed by the straight line OX passing through thepoint F pointing in the direction from point O to point F and by thetangent FY, at point F, to the projection (C), pointing in the directioncorresponding to the movement, on the projection (C), of the centralpoint D to the peripheral point E, the said angle being taken aspositive in the considered direction of rotation.

14. Using a linguistic convention, the term "tangent", at a point of theprojection (C), is used regardless of whether the said point forms partof a curved section or of a rectilinear section of the said projection.In the case where the said point forms part of a rectilinear section ofthe said projection, the tangent to the said projection, at any point ofthe said rectilinear section, denotes the straight line which carriedthe said rectilinear section.

15. The "front face" of the arm means the front part of the arm boundedby the line of its points of meeting with the surface of the volumeswept by rotating the stirrer about the axis of the autoclave and withthe surface Σ₁.

16. The "contour" of the front face of the arm means the line of thepoints bounding the front face of the arm.

17. The "lower line" and "upper line" of the arm mean the lower part andthe upper part, respectively, of the contour of the front face of thearm, each part being bounded by its point of meeting with surface the Σ₁and by the highest point of the said contour situated on the surface Σ₂.

18. An "elevation" projection and an "elevation" view mean theprojection, in any plane (Q) passing through the axis of the autoclave,obtained by folding back, in the said plane (Q), alone the circular arcscentered on the axis of the autoclave.

19. (G) and (G') denote the respective elevation projections HJ and H'Jof the lower line and of the upper line of the arm in the plane (Q), thesaid elevation projections being called, respectively, more briefly,"projection" (G) and "projection" (G').

20. The points H and H', called the "central" points of the projections(G) and (G'), respectively, are situated at a distance from the axis ofthe autoclave which is equal to 1.05 r.

21. the point J common to the projections (G) and (G'), called the"extreme peripheral" point of the projection (G) and/or of theprojection (G'), is situated at a distance d from the axis of theautoclave.

22. h denotes the distance from the joint J to the radius of theautoclave flange, which distance is measured in the vertical directionand taken as positive from the bottom upwards starting from the radiusof the flange, taken as origin.

23. e' denotes the distance from the central point H of the projection(G) to the central point H' of the projection (G').

24. S denotes the surface area of the surface bounded by the elevationprojection of the contour of the front face of the arm.

25. (s) denotes the cross section of the surface of the arm in a planeat right angles to its leading line at any point M.

26. M', N and N' denote the points of meeting of the cross section (s)and, respectively, of the trailing line and of the lower and upper linesof the contour of the front face of the arm.

27. a₁, a₂, a₃, and a₄ denote the apexes of the rectangle, with twohorizontal sides, circumscribed on the cross section (s).

28. a₁, a₂, a₃ a₄, a₁ a₄ and a₂ a₃ denote the sides of the saidrectangle on which the points M, M', N and N' are situated,respectively, whatever the shape of the cross section (s).

29. Each of the arms of any stirrer of this type employed until now,called, in the present description, "stirrer Bo", has the followingcharacteristic: the projections (C) and (C') of its leading line and ofits trailing line, respectively, are rectilinear, parallel and arrangedso that the point O is situated between the straight lines which produceit. Whatever the considered direction of rotation of the said stirrerBo, the angle of curvature α at any point F on the projection (C) isnegative. When rotating in either of the two directions of its rotation,the prior art said stirrer Bo drives the medium with a centrifugalmotion; away from the axis, in the lower part of the autoclave.

The power consumed by the prior art stirrer Bo at certain criticalstages of the process, especially at the end of the polymerizationoperation and during the degassing treatment of the polymerizate, makesit necessary to provide the said stirrer with a relatively large driveunit.

SUMMARY OF THE INVENTION

The autoclave according to the invention does not have theabove-mentioned disadvantages.

Briefly, the present invention comprises a vertical autoclave to carryout operations involving a medium having a liquid phase and/or a solidphase comprising:

(a) a concave autoclave bottom,

(b) a shaft along a vertical axis of said autoclave, said shaft passingthrough said bottom of said autoclave, and

(c) stirrer means connected to said shaft for driving said medium with acentripetal motion about said concave autoclave bottom.

The invention also comprise the process for the bulk manufacture ofvinyl chloride-based polymers comprising carrying out the polymerizationin two stages; the second stage being carried out in the autoclave ofthe instant invention as more particularly set forth hereinafter.

DESCRIPTION OF THE DRAWINGS

FIG. 1 top plan view of a prior art stirrer arm.

FIG. 2 front elevation view of a prior art stirrer arm.

FIG. 3 side cross-sectional view of prior art stirrer arm taken alongline III--III of FIG. 1.

FIG. 4 graph showing plotted curves of power consumed by the stirrermeans of the present invention, curve (L), and the prior art stirrer Bocurve (Lo), where the ordinate is power consumed (KW) and the abscissais the rate of rotation (rev/min).

FIG. 5 partial cross-sectional view of the autoclave of the presentinvention with stirrer in place.

FIG. 6 cross-sectional view of the autoclave taken along line VI--VI ofFIG. 5.

FIG. 7 top plan view of the stirrer arm of the present invention andaxial shaft.

FIG. 8 front elevation view of stirrer arm of the present invention andaxial shaft.

FIG. 9 cross-sectional view of stirrer arm taken along line IX--IX ofFIG. 7.

FIG. 10 cross-sectional view of stirrer arm taken along line X--X ofFIG. 7.

DETAILED DESCRIPTION

According to the invention, stirrer of the present invention(hereinafter referred to as the "instant stirrer") is capable of drivingthe medium with a centripetal motion in the lower part of the autoclave.Preferably, the autoclave with a vertical axis which is the subject ofthe invention is equipped with a stirring device of the type comprisinga stirrer consisting of at least one arm matching the shape of theconcave bottom of the autoclave and connected to a shaft passing throughthe bottom of the autoclave along its axis.

Preferably, the instant stirrer is capable of driving with a centripetalmotion at least any component unit of the volume of the medium that issituated in the minimum working region of the lower part of theautoclave.

A "minimum working" region or zone of the lower part of the autoclavemeans, in the present description, the region or zone of the part of theautoclave bounded by the cylindrical surfaces of revolution, having theaxis of the autoclave as their axis, whose cross section, in a plane (P)perpendicular to axis, has a radius which is equal, in the first case,to r+0.2 d and, in the other case, to 0.9 d.

The instant stirrer is capable of fulfilling the indicated function inone of its two directions of rotation, denoted by the "appropriate"direction of rotation of the side stirrer, as hereinafter described.

It has been found, in fact, that the maximum power consumed by theinstant stirrer is when the instant stirrer is rotating in theappropriate direction of rotation at a given rate of rotation, duringthe final polymerization operation of the process for the bulkmanufacture of vinyl chloride-based polymers and copolymers in twostages carried out in separate units and during the degassing treatmentof the polymerizate obtained, is less than the maximum power consumed,everything else being equal, during the rotation of a prior art stirrerBo corresponding to the instant stirrer, in the appropriate direction ofrotation of the prior art stirrer Bo.

FIGS. 1, 2, and 3 of the drawings show views of an arm of a prior artstirrer Bo given by way of a typical example.

FIG. 1 is a plan view showing projections (C) and (C') are rectilinearand symmetrical relative to a straight line passing through O.

FIG. 2 is an elevation front view of a stirrer Bo.

FIG. 3, the section III--III is a right-angled trapezium in which theapex of the acute angle is situated on the leading line DE and on thelower line of the contour of the front face of the arm, said lines beingcoincident, and in which the large base, equal to the height, twice thelength of the small base, forms an angle of 7 degrees to the horizontal.

In the present description, the prior art stirrer Bo "corresponding" tothe instant stirrer means any stirrer Bo whose shape and dimensions aresuch that:

it comprises as many arms as the instant stirrer and their mutualangular arrangement is that of the arms of the instant stirrer or thesymmetrical arrangement by reflection,

the radius r of its shaft is equal to the radius of the shaft of theinstant stirrer, and

the distances d and h and the surface area S relative to each of itsarms are equal, respectively, to those relative to the arm of thecorresponding angular arrangement of the instant stirrer.

In the present description, the "appropriate" direction of rotation ofthe prior art stirrer Bo corresponding to the instant stirrer means thedirection of rotation in which the power consumed is the lowest,everything else being equal.

It has been found that the observed power reduction, during thedegassing of the unreacted monomer composition, in the case, forexample, of a vinyl chloride-based polymer whose apparent density liesbetween 0.55 g/cm³ and 0.65 g/cm³ and whose mean particle diameter isbetween 100 and 150 μm, is from 15% to 20%. FIG. 4 of plate 2 of theattached drawing shows, by way of example, curves showing the value ofthe power consumed (kW) plotted as the ordinate, as a function of therate of rotation (rev/min) of the instant stirrer, plotted as theabscissa, for the rotation of the said stirrer in said polymer held inan autoclave with a capacity of 36 m³. In FIG. 4, the curves (L) and(Lo) relate, respectively, to a stirrer according to the invention andto a prior art stirrer BO corresponding to the instant stirrer.

This makes it possible, at a given power, and thus without recourse to alarger stirrer drive unit, to use autoclaves of larger capacity. It hasbeen found, for example, that it is possible to carry out the finalpolymerization operation and the treatment of degassing the polymerizateobtained in an autoclave according to the instant invention, with acapacity of 65 m³, under stirring conditions such that, everything elsebeing equal, the maximum power consumed is not higher than the maximumpower consumed in an autoclave with a capacity of 50 m³ fitted with aprior art stirrer Bo.

It has also been found that the proportion by weight of the correctproduct obtained while using the instant stirrer is generally greaterthan that obtained, everything else being equal, when using a prior artstirrer Bo corresponding to the instant stirrer.

It has also been found that the turnover time of the said polymer,relative to a stirrer according to the invention, is much shorter thanthe turnover time relative to a prior art stirrer Bo corresponding tothe instant stirrer according to the invention and can be 5 timesshorter than the latter.

The turnover time is the time required to obtain a homogeneousdispersion of a colored portion of the said polymer in the remaining,uncolored, portion by rotation at a specified rate of rotation, of thestirrer considered. The quantity of the polymer used representsapproximately 0.4 t per m³ of autoclave capacity. The colored portion ofthe polymer represents 1% by weight of the quantity of polymer used. Thestirring is started up after addition of the colored polymer into theautoclave containing the uncolored polymer. The moment at which ahomogenous dispersion is obtained is defined as that when the polymercolor, observed visually, becomes uniform. The turnover time is thatwhich separates the instant when the stirring is started up from theinstant when the homogeneous dispersion is obtained.

The turnover time relative to the stirrer according to the invention isdetermined by rotating the instant stirrer in the said polymer held inthe said autoclave.

The turnover time relative to the prior art stirrer Bo is determined byrotating the said stirrer in the said polymer held in the saidautoclave, when the instant stirrer is replaced by the prior art stirrerBo.

The turnover time is that obtained by rotating the instant stirrer inthe appropriate direction of rotation.

An advantageous alternative embodiment of the instant stirrer is astirrer with an arm of a curved shape.

Referring to the drawings, a stirrer with an "arm of a curved shape"means, in the present description, a stirrer in which a central angle φ(as defined below) of the minimum working part (as defined below) of theprojection (C) of the leading line of the stirrer arm, in one of the twodirections of rotation of the said stirrer, is between +5 and +90degrees.

The "minimum working part" of the projection (C) means, in the presentdescription as shown in FIG. 7, the part K₁ K₂ of the projection (C),said part K₁ K₂ being shown in FIG. 7 as the projection, in the plane(P), of the part of the leading line situated inside the minimum workingzone (as previously defined) and bounded by its points of meeting of thesurface of the said zone.

The points K₁ and K₂ called, respectively, the "central" point and the"peripheral" point of the minimum working part of the projection (C),are situated at a distance from the point O which is equal, in the caseof the point K₁, to r+0.2 d and, in the case of point K₂, to 0.9 d.

The "central angle φ", as shown in FIG. 7, of the minimum working partof the projection (C) means, in the present description, the angle O K₁,O K₂, taken as positive in the said direction of rotation.

Depending on whether the said direction of rotation is the clockwisedirection of rotation or the anticlockwise direction of ration as shownin FIG. 7 by arrow 20 an "arm with a shape curved to the right" or an"arm with a shape curved to the left" or, more briefly, "arm curved tothe right" or "arm curved to the left".

The said direction of rotation is the "appropriate" direction ofrotation of the stirrer according to the invention; (i.e., clockwise foran arm curved to the right and counterclockwise for an arm curved to theleft).

According to an embodiment which is given by way of being a preferredone, of the arm with a curved shape, the angle of curvature α, at anypoint F on the minimum working part of the projection (C), is between 0and +90 degrees.

According to an embodiment which is given by way of being more highlypreferred, of the arm with a curved shape, the angle of curvature α atany point F on the minimum working part of the projection (C), isbetween +8 and +45 degrees.

The angle of curvature α may be variable, continually increasing orcontinually decreasing, for example, or constant, from the central pointK₁ to the peripheral point K₂ of the minimum working part of theprojection (C).

The value of the angle of curvature α at the point F determines,everything else being equal, the value, relative to said arm, of thecentripetal flowrate of the medium through the cylindrical surface ofrevolution whose axis is the axis of the autoclave and which passesthrough the point F. The centripetal flowrate is zero when the angle ofcurvature α is zero or equal to +90 degrees and is maximum when theangle of curvature α is equal to +45 degrees. It is an increasingfunction of the angle of curvature α, when the angle of curvature αincreases from 0 to +45 degrees. It is a decreasing function of theangle of curvature α when the angle of curvature increases from +45 to+90 degrees.

By way of example, the angle of curvature α can be substantiallyconstant from the central point K₁ to the peripheral point K₂ of theminimum working part of the projection (C).

"Substantially constant" is to be understood to mean that the angle ofcurvature α does not differ from an average value by more than 5degrees.

A description of a stirrer with an arm of a curved shape according tothe embodiment given by way of being a preferred one, of the said arm ofcurved shape, is given below.

The radius r is generally between 0.10 R and 0.25 R.

The distance d, at most slightly less than R, is generally at leastequal to 0.5 R.

The ratio r/d is generally between 0.1 and 0.5.

The distance h is generally between -0.65 R and +0.30 R.

The choice of the shape of the projection (C') and of its positionrelative to the projection (C) is not critical.

The distance e is generally at most equal to 2 r and preferably between0.75 r and 1.50 r.

The distance e' is generally between 0.05 R and 0.5 R.

The surface area S is generally between 0.1 R₂ and 0.5 R².

The shape of the cross section (s) is not critical. By way of example,the cross section (s), at any point M on its leading line whoseprojection in the plane (P) is a point situated on the minimum workingpart of the projection (C), is such that:

the ratio of the distances Ma₁ /a₁ a₂ is less than 0.20 and preferablyless than 0.05,

the part MN' of the said cross section, situated on the front face ofthe stirrer arm, has, essentially, a substantially rectilinear profileforming, with the side a₁ a₂, an angle which is zero or at most equal to45 degrees at the point on the leading line whose projection in theplane (P) is the central point K₁ of the minimum working part of theprojection (C), and gradually decreasing, when the point M moves alongthe leading line in the direction corresponding to the movement from thecentral point K₁ to the peripheral point K₂ of the minimum working partof the projection (C), to a value which is eventually zero,

the ratio of the distances Na₁ /a₁ a₄ is less than 0.20 and preferablyless than 0.15, and

the part NM' of the said cross section, situated on the face, visiblefrom below, of the stirrer arm, has essentially a substantiallyrectilinear profile forming, with the side a₁ a₄ an angle γ of between 3and 10 degrees and preferably between 5 and 10 degrees.

For reasons of a mechanical nature, the cross section (s) is such that,when the point M moves along the leading line in the directioncorresponding to the movement from the central point D to the peripheralpoint E of the projection (C), the length of the sides of the rectanglewith two horizontal sides, circumscribed on the cross section (s), ispreferably decreasing.

The instant stirrer advantageously comprises several arms, preferablyidentical in shape, and their number not being limited. In the casewhere the instant stirrer comprises several arms, each of them isdesigned and arranged so that the appropriate direction of rotation ofeach of the instant stirrers, which comprised each of the arms takenseparately, is the same for all the said instant stirrers. The said armsare generally arranged in a uniform manner around the axis of theautoclave. Thus, in the case of two arms, these are preferably arrangedsymmetrically relative to the axis of the autoclave; when the instantstirrer has three arms, these form a mutual angle of 120 degrees, takenpair-by-pair.

The stirring device in the autoclave according to the invention can alsocomprise at least one other stirrer, the other stirrer or the otherstirrers and the instant stirrer being controlled independently. Theother stirrer may have a rotary shaft passing through the top part ofthe autoclave. The stirring device can comprise a single other stirrer,with a rotary shaft directed, preferably, along the axis of theautoclave. It may also comprise several other stirrers arranged,preferably, in a regular manner around the axis of the autoclave.

The other stirrer may be of a type which essentially consists of a bandwound in helical turns about the said rotary shaft. It is then capableof driving the reaction medium with a rising motion in one of the twodirections or rotation, denoted by the "appropriate" direction ofrotation of the said stirrer. Depending on the design of the instantstirrer and that of the other stirrer, the appropriate direction ofrotation of the other stirrer is the same as that of the instant stirreror the direction opposite to the appropriate direction of rotation ofthe instant stirrer. The other stirrer may be, in particular, such asdescribed in French Patent No. 75/32,124, published under No. 2,328,722.

The instant stirrer and the other stirrer may be hollow and may carry acoolant fluid.

A description of an embodiment of an autoclave according to theinvention is given below, by way of example, with reference to FIGS. 5and 10 of the drawings.

The autoclave is fitted with a stirring device comprising the instantstirrer with two arms curved to the left and another stirrer, with arotary shaft passing through the upper part of the autoclave along itsaxis, the stirrer according to the invention and the other stirrer beingcontrolled independently.

The autoclave 1, with a vertical axis, is surrounded by jacket 2 inwhich a heat exchange fluid which enters through a pipe 3 and leavesthrough a pipe 4 is circulated. In its upper part, the autoclave 1comprises a pipe 5 for charging the prepolymerizate, the requiredreactants and, if appropriate, monomers, and a pipe 6 for removing theunreacted monomer composition at the end of the operation. In its lowerpart, the autoclave 1 has a pipe 7 for discharging the polymer.

Passing through the bottom 8 of the autoclave 1 is a shaft 9 to whichthere are attached two arms 10 arranged symmetrically relative to theaxis of the autoclave, and which is supported by the bearing 11, thesealing between the shaft 9 and the bottom of the autoclave beingensured by the sealing device 12 consisting of a stuffing box or amechanical seal.

Passing through the upper part of the autoclave 1 along the axis of theautoclave is a rotary shaft 13 supported by the bearing 14, the sealingbetween the rotary shaft and the upper part of the autoclave beingensured by means of a sealing device (stuffing box or mechanical seal)15. A band 16 of the type described in French Patent No. 75/32,124,published under No. 2,328,722 is wound in helical turns on the rotaryshaft 13.

The autoclave according to the invention is especially suitable forimplementing the final polymerization or copolymerization operation inthe process for the bulk manufacture of vinyl chloride-based polymersand copolymers in two stages which are carried out in separate unit.

According to an alternative embodiment of the process of the invention,a polymerization or copolymerization operation is carried out in a firststage with high-turbulence stirring, on a monomer composition based onvinyl chloride, until a degree of conversion of the monomer compositionof 3% to 15% is obtained, an additional quantity of a monomercomposition based on vinyl chloride, which is identical to or differentfrom that used during the first stage is added to the reaction medium,if appropriate, and then a final polymerization or copolymerizationoperation is carried out during the second stage, on the reaction mediumthus formed, at a slow rate of stirring in the autoclave according tothe invention.

According to the said alternative embodiment, during the second stage,the reaction medium is subjected to the action of a stirrer with arotary shaft passing through the upper part of the autoclave anddirected along the axis of the autoclave, consisting essentially of aband wound in helical turns on the said rotary shaft, the said stirrerforming a stirring zone around the axis of the autoclave, and to theaction of the instant stirrer forming, simultaneously and independently,another stirring zone in the lower part of the autoclave.

According to the said alternative embodiment, the reaction medium isdriven with a centripetal motion by the instant stirrer, in the lowerpart of the autoclave.

Any polymerization initiators capable of being employed for the bulkmanufacture of vinyl chloride-based polymers and copolymers may be usedaccording to the process of the invention and, in general, free radicalgenerators such as organic peroxides such as lauroyl peroxide, acetylcyclohexanesulphonyl peroxide, isobutyryl peroxide, dichloroacetylperoxide, trichloroacetyl peroxide; peroxydicarbonates such as ethylperoxydicarbonate, ethylhexyl peroxydicarbonate, isopropylperoxydicarbonate or isobutyl peroxydicarbonate; tert-butylperoxymethoxyacetate; tert-butylperoxyethoxyacetate; tert-butylperoxy2-phenoxypropionate; and azo compounds such as 2,2'-azobis(2,4dimethylvaleronitrile). In general 0.001 to 0.006% by weight relative tothe total monomer composition employed, of the polymerization initiatoror initiators are used, expressed as active oxygen.

The polymerization temperature is generally between 10° and 80° C. andpreferably between 30° and 70° C.

The invention will be further described in connection with the followingexamples which are set forth for purposes of illustration only.

Examples 1, 3, 5, 7, 9, 11, 12, and 14 are according to the instantinvention.

Examples 2, 4, 6, 8, 10, 13, and 15 are given by way of comparison.

For each of the autoclaves, called polymerizers, of a given capacity andused in the examples and fitted with a stirring device comprising theinstant stirrer or a prior art stirrer Bo, the characteristics of thesaid stirrers are shown in Table I.

The said autoclaves are made of stainless steel and fitted with ajacket. The stirring device also comprises another stirrer consisting ofa band wound in helical turns on a rotary shaft passing through theupper part of the autoclave along its axis, as described in FrenchPatent No. 75/32,124, published under No. 2,328,722.

For each stirring device, the appropriate direction of rotation of theother stirrer is the direction opposite to the appropriate direction ofrotation of the instant stirrer or of the corresponding prior artstirrer Bo.

In all the examples the direction of rotation of each stirrer is theappropriate direction of rotation of the said stirrer.

Examples 1 and 11 relate to the bulk manufacture, in two stages, carriedout in separate units, of vinyl chloride-based polymers or copolymers.In each of the said examples, the second polymerization stage is carriedout in one of the said autoclaves.

The correct product is defined as that which passes through a sieve witha mesh opening of 315 μm.

The viscosity index of the vinyl chloride-based polymers and copolymersis determined in accordance with the international standars ISO 174.

EXAMPLES 1 and 2

135 kg of vinyl chloride are introduced into a prepolymerizer 200 litersin capacity, made of stainless steel and fitted with a stirrerconsisting of a "lightnin" turbine with 6 flat paddles, and theapparatus is purged by degassing with 10 kg of vinyl chloride. 16.6 g ofethyl peroxydicarbonate are also added, corresponding to 1.5 g of activeoxygen. The rate of stirring is set at 500 rev/min.

The temperature of the reaction medium in the prepolymerizer is raisedand maintained at 71° C., which corresponds to a relative pressure of 12bars in the prepolymerizer.

After 15 minutes' prepolymerization, the degree of conversion being inthe region of 8%, the prepolymerizate is transferred to the verticalpolymerizer 0.4 m³ in capacity, purged beforehand by degassing with 20kg of vinyl chloride and containing 130 kg of vinyl chloride, 22.25 g ofethyl peroxydicarbonate, corresponding to 2 g of active oxygen, and 75 gof lauroyl peroxide, corresponding to 3 g of active oxygen. The rate ofrotation of the instant stirrer (Example 1) or of the prior art stirrerBo (Example 2) is set at 30 rev/min. and that of the other stirrer at 50rev/min. The temperature of the reaction medium is raised over 15minutes, and then maintained at 69° C., which corresponds to a relativepressure of 11.5 bars in the polymerizer. After reaching thetemperature, the polymerization in the polymerizer takes 3.5 hours.

Degassing of the unreacted monomer is then carried out. After degassingand breaking the vacuum with nitrogen, the polyvinyl chloride obtainedis collected.

EXAMPLES 3 and 4

The prepolymerization is carried out as in Example 1.

After 15 minutes' prepolymerization, the degree of conversion being inthe region of 8%, the prepolymerizate is transferred to the verticalpolymerizer 0.4 m³ in capacity, purged beforehand by degassing with 20kg of vinyl chloride and containing 130 kg of vinyl chloride, 27.8 g ofacetylcyclohexanesulphonyl peroxide, corresponding to 2 g of activeoxygen, and 22.25 g of ethyl peroxydicarbonate, corresponding to 2 g ofactive oxygen. The rate of rotation of the instant stirrer (Example 3)or of the prior art stirrer Bo (Example 4) is set at 30 rev/min and thatof the other stirrer at 50 rev/min. The temperature of the reactionmedium is raised over 15 minutes, and then maintained at 55° C., whichcorresponds to a relative pressure of 8.1 bars in the polymerizer. Afterthe temperature has been reached, the polymerization in the polymerizertakes 4 hours.

Degassing of the unreacted monomer is then carried out. After degassingand breaking the vacuum with nitrogen the polyvinyl chloride obtained iscollected.

EXAMPLES 5 and 6

133 kg of vinyl chloride are introduced into the prepolymerizer used inExample 1 and the apparatus is purged by degassing with 10 kg of vinylchloride. 2 kg of vinyl acetate, 5.56 g of acetyl cyclohexanesulphonylperoxide, corresponding to 0.4 g of active oxygen, and 13.35 g of ethylperoxydicarbonate, corresponding to 1.2 g of active oxygen, are alsoadded. The rate of stirring is set at 500 rev/min.

The temperature of the reaction medium in the prepolymerizer is raisedand maintained at 70° C., which corresponds to a relative pressure of11.3 bars in the prepolymerizer.

After 15 minutes' prepolymerization, the degree of conversion being theregion of 8%, the prepolymerizate is transferred to the verticalpolymerizer 0.4 m³ in capacity, purged beforehand by degassing with 15kg of vinyl chloride and containing 125 kg of vinyl chloride, 5 kg ofvinyl acetate and 76.4 g of acetyl cyclohexanesulphonyl peroxide,corresponding to 5.5 g of active oxygen. The rate of rotation of theinstant stirrer (Example 5) or of the prior art stirrer Bo (Example 6)is set at 30 rev/min and that of the other stirrer at 50 rev/min. Thetemperature of the raction medium is raised over 15 minutes, and thenmaintained at 47° C., which corresponds to a relative pressure of 6.2bars in the polymerizer. After reaching the temperature, thepolymerization in the polymerizer takes 6.5 hours.

Degassing of the unreacted monomer composition is then carried out.After degassing and breaking the vacuum with nitrogen, a copolymer ofvinyl chloride and vinyl acetate consisting of 98% by weight of vinylchloride and 2% by weight of vinyl acetate is collected.

EXAMPLES 7 and 8

12.5 t of vinyl chloride are introduced into a prepolymerizer 20 m³ incapacity made of stainless steel and fitted with a stirrer comprising aship's propeller above which a turbine with 6 flat paddles is mounted,and the apparatus is purged by degassing with 1 t of vinyl chloride.1.79 kg of ethyl peroxydicarbonate, corresponding to 160 g of activeoxygen, are also added. The rate of stirring is set at 120 rev/min.

The temperature of the reaction medium in the prepolymerizer is raisedand maintained at 67° C., which corresponds to a relative pressure of 11bars in the prepolymerizer.

After 15 minutes' prepolymerization, the degree of conversion being inthe region of 8%, the prepolymerizate is transferred to the verticalpolymerizer 36 m³ in capacity, purged beforehand by degassing with 0.5 tof vinyl chloride and containing 7.5 t of vinyl chloride, 1.33 kg ofethyl peroxydicarbonate, corresponding to 120 g of active oxygen, and4.48 kg of lauroyl peroxide, corresponding to 180 g of active oxygen.The rate of rotation of the instant stirrer (Example 7) or of the priorart stirrer Bo (Example 8) is set at 20 rev/min and that of the otherstirrer at 30 rev/min. The temperature of the reaction medium is raisedover 30 minutes, and then maintained at 70° C., which corresponds to arelative pressure of 11.9 bars in the polymerizer. After reaching thetemperature, polymerization in the polymerizer takes 3.5 hours.

Degassing of the unreacted monomer is then carried out. After degassingand breaking the vacuum with nitrogen, the polyvinyl chloride obtainedis collected.

EXAMPLES 9 and 10

18 t of vinyl chloride are introduced into a prepolymerizer 30 m³ incapacity, made of stainless steel and fitted with a stirring devicecomprising a ship's propeller above which a turbine with 6 flat paddlesis mounted, and the apparatus is purged by degassing with 1 t of vinylchloride. 2.56 kg of ethyl peroxydicarbonate, corresponding to 230 g ofactive oxygen, are also added. The rate of stirring is set at 100rev/min.

The temperature of the reaction medium in the prepolymerizer is raisedand maintained at 67° C., which corresponds to a relative pressure of 11bars in the prepolymerizer.

After 15 minutes' prepolymerization, the degree of conversion being inthe region of 8%, the prepolymerizate is transferred to the verticalpolymerizer 50 m³ in capacity, purged beforehand by degassing with 0.5 tof vinyl chloride and containing 11.5 t of vinyl chloride, 1.89 kg ofethyl peroxydicarbonate, corresponding to 170 g of active oxygen, and6.47 kg of lauroyl peroxide, corresponding to 260 g of active oxygen.The rate of rotation of the instant stirrer according to the invention(Example 9) or of the prior art stirrer Bo (Example 10) is set at 15rev/min and that of the other stirrer at 25 rev/min. The temperature ofthe reaction medium is raised over 30 minutes, and then maintained at70° C., which corresponds to a relative pressure of 11.9 bars in thepolymerizer. After reaching the temperature, the polymerization in thepolymerizer takes 3.5 hours.

Degassing of the unreacted monomer is then carried out. After degassingand breaking the vacuum with nitrogen, the polyvinyl chloride obtainedis collected.

EXAMPLE 11

23.5 t of vinyl chloride are introduced into a prepolymerizer 40 m³ incapacity, made of stainless steel and fitted with a stirring devicecomprising a ship's propeller above which a turbine with 6 flat paddlesis mounted, and the apparatus is purged by degassing with 1.5 t of vinylchloride. 3.34 kg of ethyl peroxydicarbonate, corresponding to 300 g ofactive oxygen, are also introduced. The rate of stirring is set at 65rev/min.

The temperature of the reaction medium in the prepolymerizer is raisedand maintained at 68° C., which corresponds to a relative pressure of11.5 bars in the prepolymerizer.

After 15 minutes' prepolymerization, the degree of conversion being inthe region of 8%, the prepolymerizate is transferred to the verticalpolymerizer 65 m³ in capacity, purged beforehand by degassing with 1 tof vinyl chloride and containing 15 t of vinyl chloride, 2.45 kg ofethyl peroxydicarbonate, corresponding to 220 g of active oxygen, and8.46 kg of lauroyl peroxide, corresponding to 340 g of active oxygen.The rate of rotation of the instant stirrer is set at 10 rev/min andthat of the other stirrer at 23 rev/min. The temperature of the reactionmedium is raised over 30 minutes, and then maintained at 70° C., whichcorresponds to a relative pressure of 11.9 bars in the prepolymerizer.After reaching the temperature, the polymerization in the polymerizertakes 3.5 hours.

Degassing of the unreacted monomer is then carried out. After degassingand breaking the vacuum with nitrogen, the polyvinyl chloride obtainedis collected.

Tables II and III show, respectively, for each of the Examples 1 to 6and 7 to 11:

(i) the capacity of the autoclave used as polymerizer, referred toalready;

(ii) the maximum power consumed during the polymerization or thedegassing treatment;

(iii) the quantity of polymer collected; and

(iv) the content by weight of the correct product.

They also indicate:

(i) the viscosity index;

(ii) the apparent density; and

(iii) the mean particle diameter, determined for the correct product.

EXAMPLES 12 and 15

These relate to the determination of the power consumed and of theturnover time relative to the instant stirrer (Examples 12 and 14) andto the prior art stirrer Bo (Examples 13 and 15), for the rotation ofthe stirrer in a vinyl chloride-based polymer held in the autoclave. Theapparent density of the said polymer is 0.60 g/cm³ and its mean particlediameter is 120 μm.

Table IV shows, for each of the examples, the autoclave capacity, thequantity of polymer used, the rate of rotation of the instant stirrer orof the corresponding prior art stirrer Bo, the power consumed, and theturnover time.

                  TABLE I                                                         ______________________________________                                        Characteristics of the instant stirrers                                       and of the corresponding prior art stirrers Bo                                ______________________________________                                        Autoclave capacity (m.sup.3)                                                                  0.4     36       50    65                                     ______________________________________                                        Characteristics common                                                        to the instant stirrer                                                        and to the correspond-                                                        ing prior art stirrer                                                         Bo                                                                            Number of arms  2       2        2     2                                      Angular distancs                                                              between arms                                                                  (degrees)       180     180      180   180                                    r/R             0.11    0.13     0.12  0.13                                   d/R             0.98    0.89     0.85  0.95                                   r/d             0.12    0.14     0.15  0.24                                   h/R             +0.25   -0.22    -0.17 -0.1                                   S/R.sup.2       0.14    0.13     0.15  0.1                                    ______________________________________                                        Characteristics                                                               specific to the                                                               instant stirrer                                                               .0. (degrees)   +12     +9       +7    +6                                     α (degrees)*                                                                            +10 to  +10 to   +10 to                                                                              +10 to                                                 +30     +25      +25   +25                                    e/r             1.2     1.1      1.1   1.1                                    e'/R            0.15    0.17     0.17  0.18                                   Ma.sub.1 /a.sub.1 a.sub.2                                                                     0.01    0.01     0.01  0.01                                   Value of a                                                                    point K.sub.1                                                                 (degrees)       25      25       25    25                                     Minimum value                                                                 of β (degrees)                                                                           0       0        0     0                                      Na.sub.1 /a.sub.1 a.sub.4                                                                     0.08    0.1      0.1   0.1                                    γ degrees)                                                                              9       7        7     7                                      ______________________________________                                        Characteristics                                                               specific to the                                                               prior art stirrer                                                             Bo corresponding                                                              to the instant                                                                stirrer                                                                       e/r             0.9     1.4      1.3   --                                     e'/R            0.05    0.18     0.22  --                                     The remaining characteristics are those of the stirrer                        shown in FIGS. 1, 2, and 3.                                                   ______________________________________                                         *at any point F on the part K.sub.1 K.sub.2                              

                                      TABLE II                                    __________________________________________________________________________                  Example     Example     Example                                               accord-     accord-     accord-                                               ing to      ing to      ing to                                                the in-                                                                            Comparative                                                                          the in-                                                                            Comparative                                                                          the in-                                                                            Comparative                                      vention                                                                            example                                                                              vention                                                                            example                                                                              vention                                                                            example                                          1    2      3    4      5    6                                  __________________________________________________________________________    Capacity of the auto-                                                                       0.4  0.4    0.4  0.4    0.4  0.4                                clave used as polymerizer                                                     (m.sup.3)                                                                     Maximum power consumed                                                                      6    7.5    6.1  7.8    5.7  7.1                                (kW)                                                                          Quantity of polymer                                                                         180  180    160  160    200  200                                collected (kg)                                                                Weight content of                                                                           93   92     94   93     88   85                                 correct product (%)                                                           Viscosity index                                                                             78   78     108  108    142  142                                Apparent density                                                                            0.58 0.58   0.57 0.56   0.50 0.50                               (g/cm.sup.3)                                                                  Mean particle diameter                                                                      110  110    130  130    120  120                                (μm)                                                                       __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________                  Example     Example     Example                                               accord-     accord-     accord-                                               ing to      ing to      ing to                                                the in-                                                                            Comparative                                                                          the in-                                                                            Comparative                                                                          the in-                                               vention                                                                            example                                                                              vention                                                                            example                                                                              vention                                               7    8      9    10     11                                      __________________________________________________________________________    Capacity of the autoclave                                                                   36   36     50   50     65                                      used as polymerizer (m.sup.3)                                                 Maximum power consumed                                                                      105  126    132  160    160                                     (kW)                                                                          Quantity of polymer                                                                         12.7 12.5   19.1 19     23                                      collected (t)                                                                 Weight content of                                                                           98   96     98   96     98                                      correct product (%)                                                           Viscosity index                                                                             78   78     78   78     79                                      Apparent density (g/cm.sup.3)                                                               0.60 0.60   0.60 0.60   0.60                                    Mean particle diameter (μm)                                                              105  105    100  100    110                                     __________________________________________________________________________

                                      TABLE IV                                    __________________________________________________________________________                  Example     Example                                                           accord-     accord-                                                           ing to      ing to                                                            the in-                                                                            Comparative                                                                          the in-                                                                            Comparative                                                  vention                                                                            example                                                                              vention                                                                            example                                                      12   13     14   15                                             __________________________________________________________________________    Autoclave capacity (m.sup.3)                                                                0.4  0.4    36   36                                             Quantity of polymer used (t)                                                                0.16 0.16   15   15                                             Rate of rotation of the                                                                     30   30     20   20                                             instant stirrer or of                                                         the corresponding                                                             stirrer Bo (rev/min)                                                          Power consumed (kW)                                                                         0.6  0.75   115  130                                            Turnover time (s)                                                                           16   70     24   120                                            __________________________________________________________________________

While the invention has been described in connection with a preferredembodiment, it is not intended to limit the scope of the invention tothe particular form set forth, but, on the contrary, it is intended tocover such alternatives, modifications, and equivalents as may beincluded within the spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. A process for the bulk polymerization of a vinylchloride-based monomer reaction medium to form a vinyl chloride-basedpolymer or copolymer comprising carrying out said polymerization in twostages, the initial stage being carried out with high-turbulencestirring until a degree of conversion of the monomer reaction medium ofabout 3% to 15% is obtained, and the final stage completing thepolymerization or copolymerization to the degree desired being carriedout at a low stirring rate in an autoclave comprising a concave shapedautoclave bottom, a shaft along a vertical axis of said autoclave, saidshaft passing through said bottom of said autoclave, and at least onestirrer arm attached to said vertical shaft for driving said medium witha centripetal motion about said concave autoclave bottom said stirrerarm having a shape matching the shape of said concave bottom of saidautoclave and being attached so that said arm is adjacent said concavebottom and said final stage having two stirring zones, a first stirringzone around the axis of the autoclave and a second stirring zone in thelower portion of the autoclave in which the monomer reaction mediumpartially converted into polymer is driven in a centripetal motion. 2.The process of claim 1, wherein the vinyl chloride-based monomer in thereaction medium is vinyl chloride.